1. Field of the Invention
The present invention relates to SAW resonator filters used as band filters and other filters, for example, in mobile communications equipment. More specifically, the present invention relates to edge reflection type longitudinally coupled SAW resonator filters, each of which has a multiple-stage structure in which a single surface acoustic wave substrate is used to define a plurality of longitudinally coupled resonator filter units of an edge reflection type.
2. Description of the Related Art
There have been proposed various surface acoustic wave filters as band filters used in mobile communications equipment. Among such surface acoustic wave filters, longitudinally coupled SAW resonator filters using longitudinal-mode couplings have filter characteristics achieved by coupling two kinds of resonant modes including symmetric modes and asymmetric modes generated by interdigital transducers (hereinafter referred to as IDTs).
In SAW resonator filters, oscillations caused by other modes such as higher modes occur outside of a pass band as spurious responses. Such spurious responses appear in a different manner depending on the configurations of IDTs aligned in a surface acoustic wave propagating direction.
When attenuation outside of the pass band is not sufficiently increased in a SAW resonator filter, a method for connecting a plurality of SAW resonator filters in series is used. However, when the configurations of the plurality of the connected SAW resonator filters are exactly the same, frequency characteristics in the SAW resonator filter units are equal. As a result, since the frequency positions where the above-described spurious responses occur are the same, the spurious responses cannot be significantly attenuated.
Thus, in order to solve the above problems, a SAW resonator filter, which is shown in FIG. 6 of the present application, is disclosed in Japanese Unexamined Patent Publication No. 9-294049.
In a SAW resonator filter 51, two SAW resonator filter units 53 and 54 are disposed on a surface acoustic wave substrate 52. In the SAW resonator filter unit 53, two IDTs 55 and 56 are aligned in a surface acoustic wave propagating direction. In addition, reflectors 57 and 58 are disposed in the surface acoustic wave propagating direction on both sides of the region where the IDTs 55 and 56 are disposed.
Similar to the SAW resonator filter unit 53, the SAW resonator filter unit 54 is arranged such that reflectors 61 and 62 are disposed on both sides of the region where IDTs 59 and 60 are disposed.
The IDT 56 and the IDT 58 are electrically connected by a connecting conductive portion 63. However, the number of pairs of electrode fingers of the SAW resonator filter unit 53 is different from that of the SAW resonator filter unit 54.
In other words, in the SAW resonator filter 51, although the two longitudinally coupled SAW resonator filter units 53 and 54 are connected in series, the numbers of pairs of the electrode fingers of the IDTs of the SAW resonator filter units 53 and 54 are different. As a result, in this structure, the frequency position of a spurious response outside of the band exhibited in the frequency characteristics of the SAW resonator filter unit 53 is different from that of a spurious response outside of the band exhibited in the frequency characteristics of the SAW resonator filter unit 54. Therefore, it is possible to reduce the magnitude of a spurious response in the frequency characteristics of the overall SAW resonator filter 51.
The above-described SAW resonator filter 51, which is disclosed in Japanese Unexamined Patent Publication No. 9-294049, utilizes a Rayleigh wave as one of the surface acoustic waves. Recently, an edge reflection type surface acoustic wave device utilizing an SH-type surface acoustic wave such as a BGS (Bleustein-Gulyaev-Shimizu) wave or a Love wave, as alternatives to the Rayleigh wave, have been studied and developed. In other words, in the edge reflection type surface acoustic wave equipment, reduction in the size of the equipment can be achieved, since it is not necessary to include reflectors on both sides of IDTs.
Accordingly, when a SAW resonator filter defined by connecting the above-described two SAW resonator filter units in series is disposed in the edge reflection type surface-acoustic-wave equipment, a compact SAW resonator filter can be obtained.
However, it is significantly difficult to produce a plurality of edge reflection type SAW resonator filter units by using a single surface acoustic wave substrate and make the numbers of pairs of electrode fingers of IDTs of the SAW resonator filter units different, as described in Japanese Unexamined Patent Publication No. 9-294049. In other words, although it is necessary to form an end surface for reflecting a surface acoustic wave on both sides of IDTs in the edge reflection type SAW resonator, when the numbers of pairs of the electrode fingers of the IDTs of the plurality of the SAW resonator filter units are different, the positions of the end surfaces of the SAW resonator filter units are different from each other.
Therefore, with these conventional devices, processing of the surface acoustic wave substrate is very difficult and a precision in forming the end surfaces is reduced. As a result, the filter characteristics are greatly deteriorated.